Aqueous colloidal dispersion of a compound of cerium and at least one other element chosen from among the rare earths, transition metals, aluminum, gallium and zirconium preparation process and use

ABSTRACT

The invention relates to an aqueous colloidal dispersion of a compound of cerium and at least one other element M chosen from among the rare earths other than cerium; titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, aluminium, gallium and zirconium.  
     This dispersion is characterized in that it displays a conductivity of at most 5 mS/cm. It is obtained by a process in which there is reacted with a base a mixture of at least one salt of cerium with at least one salt of an element M mentioned above and in the presence of an acid in a quantity such that the atomic ratio H + /(Ce+M) is greater than 0.1, then the precipitate from the preceding reaction is re-dispersed in water.  
     The dispersions of this type cane be used in catalysis, in lubrication, in ceramics, in the manufacture of luminophorous compounds, in cosmetics and as anticorrosion agents.

[0001] The present invention relates to an aqueous colloidal dispersionof a compound of cerium and at least one other element chosen from amongthe rare earths other than cerium; titanium, vanadium, chromium,manganese, iron, cobalt, nickel, copper, zinc, aluminium, gallium andzirconium.

[0002] Cerium sols, quite particularly the sols of tetravalent cerium,are well known. Moreover, cerium sols in combination with anotherelement can be of great benefit, for example for cosmetics applicationsor in the field of luminophores, and in particular those likely tocontain trivalent cerium. However, in these applications, sols areneeded which are concentrated and which are pure.

[0003] The object of the invention is to resolve such difficulties andthus to obtain concentrated and pure sols, likely in particular tocontain trivalent cerium.

[0004] The invention thus relates to an aqueous colloidal dispersion ofa compound of cerium and at least one other element chosen from amongthe rare earths other than cerium; titanium, vanadium, chromium,manganese, iron, cobalt, nickel, copper, zinc, aluminium, gallium andzirconium, characterized in that it displays a conductivity of at most 5mS/cm.

[0005] The invention also relates to a process for preparing such acolloidal dispersion which is characterized in there is reacted with abase a mixture of at least one salt of cerium with at least one salt ofan element M mentioned above with a base and in the presence of an acidin a quantity such that the atomic ratio H⁺/(Ce+M) is greater than 0.1,then the precipitate from the preceding reaction is re-dispersed inwater.

[0006] Other characteristics, details and advantages of the inventionwill appear more fully upon reading of the following description, andfrom the various specific but non-limitative examples intended toillustrate it.

[0007] In the remainder of the description, the term colloidaldispersion or sol of a compound of cerium and another element mentionedabove means any system constituted of fine solid particles of colloidaldimensions based on oxide and/or hydrated oxide (hydroxide) of ceriumand the other element, in suspension in an aqueous liquid phase, thesaid types also being able, if necessary, to contain residual quantitiesof bound or adsorbed ions such as for example acetates, citrates,nitrates, chlorides or ammoniums. The percentage of bound ions X or ifnecessary X+Y, expressed as molar ratio X/ce or (X+Y)/Ce can vary forexample between 0.01 and 1.5, more particularly between 0.01 and 1. Itwill be noted that in such dispersions the cerium and the other elementcan be found either totally in the form of colloids, or simultaneouslyin the form of ions or poly-ions and in the form of colloids.

[0008] By rare earth is meant the elements of the group constituted byyttrium and the elements of the periodic classification having an atomicnumber of between 57 and 71 inclusive.

[0009] A first characteristic of the dispersion of the invention is itspurity. This purity is measured by the conductivity of the dispersion.This conductivity is at most 5 mS/cm. It can be below this value andthus be at most 2 mS/cm and preferably at most 1 mS/cm. Moreparticularly, it can be below 0.3 mS/cm.

[0010] According to another characteristic, the dispersion of theinvention has a concentration of at least 50 g/l. This concentration isexpressed as oxide and taking into account the sum of the oxides ofcerium and of the other element or elements mentioned above. Thisconcentration can be more particularly at least 80 g/l.

[0011] Another characteristic of the dispersion is that it can containcerium in oxidation state III. In this case, the level of cerium III isgenerally at most 50%. It is expressed here and for the whole of thedescription by the atomic ratio CeIII/total Ce. The level of cerium IIIcan more particularly be at most 35%. Moreover, it is preferably atleast 0.5%.

[0012] The dispersions of the invention are particularly pure in nitrateanions. More precisely, the nitrate anions content of the dispersions,measured by the nitrate anions content by weight of the colloidalparticles, is below 80 ppm. The dispersions of the invention are alsopure as regards their chloride ions content.

[0013] The quantity of element M is generally at most 50%, this quantitybeing expressed by the ratio moles of element M/sum of the moles ofelement M and of cerium. The element M can be present in differentstates of oxidation. The invention applies of course to dispersionscontaining several elements M.

[0014] The dispersions of the invention can also have a high pH, forexample between 5 and 8. These pH values, close to neutral, permitinteresting applications of the dispersions of the invention.

[0015] The colloidal particles which constitute the sols of theinvention are fine. Thus, they can have an average diameter which may bein particular between 2 and 6 nm. This diameter is determined byphotometric calculation from a HRTEM (High Resolution TransmissionElectron Microscope) analysis.

[0016] The process for preparing the dispersions of the invention willnow be described.

[0017] This process comprises a first stage in which there is reactedwith a base a mixture of at least one salt of cerium with at least onesalt of an element M. The starting point can be in particular a salt ofcerium III or a mixture comprising a salt of cerium IV plus the salt ofcerium III.

[0018] Products of the hydroxide type in particular may be used as base.Alkali or alkaline-earth hydroxides and ammonia may be mentioned.Secondary, tertiary or quaternary amines may also be used. However,amines and ammonia may be preferred insofar as they reduce the risks ofpollution by alkali or alkaline-earth cations. Urea may also bementioned.

[0019] There may be used more particularly as salts of cerium III theacetate, the chloride or the nitrate of cerium III as well as mixturesof these sols such as mixed acetates/chlorides. The nitrate of cerium IVcan be used for cerium IV, and the chlorides and the nitrates inparticular for the other elements. Salts of the same type can be usedfor the other element or elements M.

[0020] According to a specific characteristic of the process of theinvention, the reaction of the salt of cerium with the base takes placein the presence of an acid.

[0021] There may be mentioned, as acids likely to be used, the mineralacids and more particularly those corresponding to the salts of cerium,in particular of cerium III, used in the reaction. There may also bementioned in particular acetic acid, nitric acid or hydrochloric acid.

[0022] It should be noted that the acid can also be contributed by thesolution of a salt in which it is incorporated. For example, there maybe used as starting solution a solution of acid titanium chloride suchas TiOCl₂. 2HCl.

[0023] The quantity of acid present or used during the reaction is suchthat the atomic ratio H⁺/(Ce+M) is greater than 0.1, preferably 0.25.

[0024] The reaction of the base with the salts can take placecontinuously, by which is meant a simultaneous addition of the reagentsto the reaction medium.

[0025] The pH of the reaction medium is usually between 7.6 and 9.5. Itis possible to operate in conditions such that the pH of the reactionmedium is kept constant during the reaction.

[0026] A precipitate is obtained at the end of the aforementionedreaction. This precipitate can be separated from the liquid medium byany known process such as for example by centrifugation. The precipitatethus obtained can then be re-suspended in water so as to give thedispersion of the invention. The concentration of cerium in thedispersion thus obtained is generally between 0.005M and 2M, preferablybetween 0.05M and 0.25 M.

[0027] The precipitate resulting from the reaction can advantageously bewashed. This washing can be carried out by placing the precipitate inwater then, after stirring, separating the solid from the liquid mediumby centrifugation for example. This operation can be repeated severaltimes if necessary.

[0028] According to a variant of the invention, the dispersion obtainedafter re-suspension in water can be purified and/or concentrated byultrafiltration.

[0029] The washing and the ultrafiltration can be carried out under airor in an atmosphere of air and nitrogen or also under nitrogen. Theatmosphere under which these operations take place plays a part in thetransformation of the cerium III into cerium IV.

[0030] After the re-suspension in water and after the optional washingstage and, preferably, before the concentration stage if a concentrationis employed, it may be advantageous to carry out an oxidation of thedispersion; the stability of the dispersion is thus improved. Thisoxidizing treatment can take place in two ways for example.

[0031] A first way is to keep the dispersion stirred and under air, fora period which can vary from 3 to 20 hours for example. The second wayis to add oxygenated water to the dispersion. The quantity of oxygenatedwater added is adjusted so as to obtain the CeIII/total Ce ratio,mentioned above, in the final dispersion. This oxidation with theaddition of oxygenated water is preferably carried out after a stirringof the dispersion under air for a period of more than 2 hours. Theperiod during which oxygenated air is added can be between 30 mn and 6hours.

[0032] The dispersions of the invention can be used in numerousapplications. Catalysis may be mentioned, in particular for automobilepost-combustion; in this case the dispersions are used in catalystpreparation. The dispersions can also be employed for lubrication, inceramics, the production of luminophorous compounds, in cosmetics, andin this case they may be involved in the preparation of cosmeticcompositions, in particular in the preparation of anti-UV creams. Theycan be used on a substrate as anticorrosion agents.

[0033] Examples will now be given. In these examples, conductivity ismeasured with the help of a METROHM 660 CONDUCTOMETERconductivity-measuring apparatus equipped with a TACUSSEL XE100conductivity cell. The Ce III contents are given as indicated above(atomic ratio CeIII/total Ce).

EXAMPLE 1

[0034] This example relates to an aqueous colloidal dispersion ofnanometric particles of cerium and titanium with a pH close to neutral.The following are added, accompanied by stirring: 562.8 g of Ce(CH₃COO)₃at 49.3% CeO₂ (i.e. 1.6 moles of Ce) and 125 g of TiOCl_(2.) 2HCl at3.19 mole/Kg of density 1.56 (i.e. 0.4 moles of TiO₂). This is made upto 3000 ml with demineralized water. The molar ratio H⁺/(Ce+Ti) is 0.4.

[0035] The precipitation of the solid is realized in a continuousassembly comprising:

[0036] a one-litre reactor equipped with a fat-paddle stirrer, set at400 rpm with a tank foot of 0.5 l and a control electrode;

[0037] two feed flasks containing, on the one hand, the solution ofsalts of cerium described above and, on the other hand, a 3 N ammoniasolution.

[0038] The flow rate of the solution of cerium acetate and TiOCl₂ isfixed at ca. 600 ml/h and the flow rate of the ammonia solution is 340ml/h. Thus, 2880 ml of the mixture of salts of cerium and titanium and1630 ml of 3 N ammonia have been added in 288 mn.

[0039] The pH of the reaction medium is 8.5 throughout the reaction.

[0040] A Ce+Ti precipitation yield of 47% is measured.

[0041] A precipitate is obtained which is separated by centrifugation.

[0042] By calcination at 1000° C., the precipitate is measured ascontaining 15% oxide of cerium and titanium.

[0043] The precipitate is dispersed by adding demineralized water inorder to obtain a 0.12 M Ce+Ti dispersion. This is stirred for 15 mn.Fresh centrifugation is carried out. Two successive operations are thusrealized. The cerium III content of the dispersion is ca. 60%. Thedispersion is then stirred under air atmosphere, for a night. At the endof this treatment, the cerium III content of the dispersion is 6.5%, andthe total cerium content is 17.2 g/l.

[0044] 100 ml of the 0.1 M Ce+Ti dispersion are diluted to 300 ml withdemineralized water. This is concentrated to 100 ml by ultrafiltrationusing 3 KD membranes. Three ultrafiltrations are thus carried out. Inthe last ultrafiltration, the mixture is concentrated in order to obtaina concentrated dispersion with a 5.7% concentration of oxide of ceriumand titanium. The pH is 5.4 and the conductivity 1.4 mS/cm. Theconcentration of nitrate in the colloidal dispersion is less than 80ppm. By means of MET cryometry, nanometric particles ca. 3 to 4 nm insize are observed.

[0045] The dispersion obtained is stable for at least 6 months.

EXAMPLE 2

[0046] This example relates to an aqueous colloidal dispersion ofnanometric particles of cerium and iron of neutral pH.

[0047] The following are placed in a beaker, accompanied by stirring:307 g of solution of cerium (III) nitrate at 3 M/l Ce³⁺, of density1.715 and H⁺=0.01 (i.e. 0.537 mole Ce³⁺), then 194.5 g of solution ofCe(NO₃)₄ at Ce⁴⁺=1.325 M/l, H⁺=0.7 N, of density 1.44 (i.e. 0.179 molesCe⁴⁺), then 73.8 g of Fe(NO₃)_(3.) 9H₂o at 98% (i.e. 0.18 mole),previously dissolved in a total volume of 358 ml (solution at pH 1),then 32.2 g of Prolabo concentrated acetic acid (i.e. 0.54 mole ofCH₃COOH). This is made up to 2000 ml with demineralized water. The wholeis stirred until a solution which is clear to the eye is obtained. Themixture obtained then has a concentration of ca. 0.45 M cerium and iron.

[0048] The precipitation of the solid is realized in the continuousassembly described in example 1.

[0049] Thus, 2000 ml of the solution of salt of cerium and iron and 800ml of 3 N ammonia have been added in 240 mn.

[0050] The colloidal dispersion obtained after re-dispersion under airof the precipitate is washed by ultrafiltration with demineralized waterpreviously adjusted to pH 7.5 then concentrated by ultrafiltration untila concentrated dispersion with a 10.5% concentration of oxide of ceriumand iron is obtained.

[0051] A colloidal dispersion is thus obtained which is stable for atleast 6 months vis-à-vis settling.

EXAMPLE 3

[0052] This example relates to an aqueous colloidal dispersion ofnanometric particles of cerium and lanthanum with a pH close to neutral.

[0053] The following is placed in a beaker, accompanied by stirring: 512g of cerium acetate at 49.3% CeO₂, 208 cm³ concentrated acetic acid, andmade up to 3000 ml with demineralized water.

[0054] The following are added to 2500 ml of this solution of Ce(III)acetate containing 1.225 mole of Ce: 500 cm³ of a solution of acetate ofLa(III) at 0.57 M La, i.e. 0.285 mole of La. The molar ratio H⁺/(Ce+La)is 2.0.

[0055] The precipitation of the solid is realized in the continuousassembly described in example 1.

[0056] Thus, 2670 ml of the solution of acetate of cerium-lanthanum and1971 ml of 3 N ammonia have been added in 287 mn.

[0057] The pH of the reaction medium is 8.5 throughout the reaction.

[0058] A Ce+La precipitation yield of 85% is measured.

[0059] A precipitate is obtained which is separated by centrifugation.

[0060] By calcination at 1000° C., the precipitate is evaluated at 21%oxide of cerium and lanthanum.

[0061] The precipitate is dispersed by adding demineralized water inorder to obtain a 0.15 M Ce+La dispersion. This is stirred for 15 mn.Fresh centrifugation is carried out. Two successive operations are thusrealized. The cerium III content of the dispersion is 80%. Thedispersion is then stirred under air atmosphere, for a night. At the endof this treatment, the cerium III content of the dispersion is 5%.

[0062] 100 ml of the 0.15 M Ce+La dispersion are diluted to 300 ml withdemineralized water. This is concentrated to 100 ml by ultrafiltrationusing 3 KD membranes. Three ultrafiltrations are thus carried out. Inthe last ultrafiltration, the mixture is concentrated in order to obtaina concentrated dispersion with a 15.5% concentration of oxide of ceriumand lanthanum. The pH is 5.5 and the conductivity 0.24 mS/cm. Theconcentration of nitrate ions in the colloidal dispersion is less than80 ppm. By means of MET cryometry, nanometric particles ca. 3 to 4 nm insize are observed.

[0063] A colloidal dispersion is thus obtained which is stable for atleast 6 months vis-à-vis settling.

EXAMPLE 4

[0064] This example relates to an aqueous colloidal dispersion ofnanometric particles of cerium and aluminium with a pH close to neutral.

[0065] The following are placed in a beaker, accompanied by stirring:585 g of cerium acetate at 49.3% CeO₂ (1.67 moles of Ce), 101 g ofAlCl_(2,) 9H₂O (M_(w)=241 g/mole, 0.42 mole of Al) and 103 g of 10 MHCl, and made up to 3000 ml with demineralized water. The molar ratioH⁺/(Ce+Al) is 0.5.

[0066] The precipitation of the solid is realized in the continuousassembly described in example 1.

[0067] Thus, 2440 ml of this solution of acetate of cerium-aluminium and1580 ml of 3 N ammonia have been added in 244 mn.

[0068] The pH of the reaction medium is 8.5 throughout the reaction.

[0069] A precipitation yield of 64% is measured.

[0070] A precipitate is obtained which is separated by centrifugation.

[0071] By calcination at 1000° C., the precipitate is evaluated at 8.1%oxide of cerium and aluminium.

[0072] The precipitate is dispersed by adding demineralized water inorder to obtain a 0.25 M Ce+Al dispersion. This is stirred for 15 mn.Fresh centrifugation is carried out. Two successive operations are thusrealized. The cerium III content of the dispersion is 60%. Thedispersion is then stirred under air atmosphere, for a night. At the endof this treatment, the cerium III content is 31%.

[0073] 100 ml of the 0.25 M Ce+Al dispersion are diluted to 300 ml withdemineralized water. This is concentrated to 100 ml by ultrafiltrationusing 3 KD membranes. Three ultrafiltrations are thus carried out. Inthe last ultrafiltration, the mixture is concentrated in order to obtaina concentrated dispersion with a 10.6% concentration of oxide. The pH ofthe dispersion is 6.

[0074] A colloidal dispersion is thus obtained which is stable for atleast 6 months vis-à-vis settling.

1—Aqueous colloidal dispersion of a compound of cerium and at least oneother element M chosen from among the rare earths other than cerium;titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper,zinc, aluminium, gallium and zirconium, characterized in that itdisplays a conductivity of at most 5 mS/cm. 2—Dispersion according toclaim 1, characterized in that it has a concentration of cerium oxideand of at least one other element mentioned above of at least 50 g/l,more particularly of at least 80 g/l. 3—Dispersion according to claim 1or 2, characterized in that it contains cerium III. 4—Dispersionaccording to claim 3, characterized in that it has a level of cerium IIIrelative to the total cerium of at most 50%, more particularly of atmost 35%. 5—Dispersion according to one of the preceding claims,characterized in that it contains a quantity of element M of at most50%, expressed by the ratio moles of element M/sum of the moles ofelement M and of cerium. 6—Dispersion according to one of the precedingclaims, characterized in that it the colloidal particles have a nitratecontent of less than 80 ppm. 7—Dispersion according to one of thepreceding claims, characterized in that it has a pH of between 5 and 8.8—Process for preparing a colloidal dispersion according to one of thepreceding claims, characterized in that there is reacted with a base amixture of at least one salt of cerium with at least one salt of anelement M mentioned above and in the presence of an acid in a quantitysuch that the atomic ratio H⁺/(Ce+M) is greater than 0.1, preferably0.25; then the precipitate from the preceding reaction is re-dispersedin water. 9—Process according to claim 8, characterized in that, afterthe re-dispersion in water of the precipitate, the dispersion obtainedis purified by ultrafiltration. 10—Process according to claim 8 or 9,characterized in that the salt of cerium is a salt of cerium III, inparticular an acetate or a chloride. 11—Process according to one ofclaims 8 to 10, characterized in that the reaction is effectedcontinuously. 12—Process according to one of claims 8 to 11,characterized in that, after the re-dispersion in water of theprecipitate, the dispersion is subjected to an oxidative treatment.13—Process according to claim 12, characterized in that the oxidizingtreatment is effected either by stirring under air of the dispersion, orby addition of oxygenated water. 14—Use of a dispersion of the typeaccording to one of claims 1 to 7 or of the type obtained by the processaccording to one of claims 8 to 13, on a substrate as an anticorrosionagent, in a cosmetic composition, in catalysis in particular forautomobile post-combustion, in lubrication or in ceramics.